1. Field of the Invention
The present invention relates to a method of manufacturing a bismuth type oxide superconductor.
2. Description of the Related Art
In recent years, an oxide superconductor (to be abbreviated as a Y type oxide superconductor hereinafter) comprising a rare earth element, an alkaline earth metal, copper, and oxygen, for example, Y-Ba-Cu-O type oxide superconductor has been developed. However, since the yttrium (Y) type oxide superconductor absorbs moisture and is degraded within a short time period, it is very difficult to put the superconductor of this type into practical use. Recently, an oxide superconductor (to be abbreviated as a Bi type oxide superconductor hereinafter) comprising bismuth, an alkaline earth metal, copper, and oxygen has been found. This Bi type oxide superconductor does not absorb moisture nor is degraded unlike the Y type oxide superconductor. Therefore, extensive studies have been made in various fields in order to put the superconductor of this type into practical use.
The oxide superconductor of a Bi-Sr-Ca-Cu-O type or the like comprising Bi, an alkaline earth metal, copper, and oxygen has a high critical temperature (T.sub.C) and therefore is expected to be used in a wide range of applications. An oxide type superconductor molded body consisting of the above oxide superconductor has conventionally been manufactured as follows. That is, an oxide, a carbonate, and the like (e.g., a combination of Bi.sub.2 O.sub.3, SrCO.sub.3, CaCO.sub.3, CuO and the like) are used as materials, and these primary materials are mixed to obtain a desired composition. The resultant mixture is preburned to prepare a composite oxide. The prepared composite oxide is milled to obtain a secondary material powder. The secondary material powder is molded into a desired shape, sintered normally at a temperature of 800.degree. to 900.degree. C., and slowly cooled in the same atmosphere at a rate of 1.degree. to 2.degree.C./min, thereby manufacturing the molded body.
Although the oxide superconductor molded body manufactured by the above conventional method slightly contains a high-temperature phase having a critical temperature (T.sub.C) close to 110 to 120K, it also contains a low-temperature phase (85-K phase) having T.sub.C close to 85K, a phase having lower T.sub.C (50 to 60K), and non-superconducting phases of a Ca-Cu-O type, a Bi-Sr-O type, and the like.
The most widely studied method is a method of obtaining a so-called high-temperature phase having T.sub.C of 110 to 120K. In order to produce the high-temperature phase, however, a heating/sintering treatment must be performed for a very long time period. In addition, critical current density (to be abbreviated as J.sub.C hereinafter) of the high-temperature phase is low.
A so-called low-temperature phase having T.sub.C of 80 to 90K and high J.sub.C can be produced by performing a heating/sintering treatment for a comparatively short time period. However, variations in units of lots are very large, and it is difficult to stably obtain a high-performance phase.